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by Rick McMullin
C++ is an object-oriented extension to the C programming language. It was developed at Bell Labs in the early 1980s and is quickly becoming the language of choice in the computer industry. Dozens of C++ compilers are available on the market today. The most common of these for PC-based systems are Borland C++, Microsoft’s Visual C++, Zortech C++, and Watcom C++. These compilers compile MS-DOS and MS Windows applications, and some of them compile code to run on OS/2 and Windows NT, as well. In addition to the number of C++ compilers that are available on DOS-based machines, a great number are also based on other hardware architectures.
Most UNIX systems have C++ compilers available from the system vendor. Linux also comes with a C++ compiler. This is the GNU C++ compiler. The GNU C++ compiler is very closely related to the GNU C compiler (GCC). In fact, since Release 2.0 of GCC, the GNU C++ compiler has been integrated with GCC. Previous to Release 2.0 of GCC, the GNU C++ compiler was a separate program known as g++. One of the major enhancements in Release 2.0 of GCC was merging these two compilers.
GCC now incorporates a C compiler, a C++ compiler, and an Objective C compiler. You will still find the g++ executable on your system, but it is now a script file that calls GCC with all the standard C++ options.
C++ and object-oriented programming (OOP) did not just happen. There were many fundamental reasons for the shift from structured programming to OOP. In the early days of computer programming, back when PDP-8s still roamed the earth in great numbers, there was a shift from machine language coding to assembler language coding. This was done because the computers of the day were a little more powerful than their predecessors. Programmers wanted to make their lives easier by moving some of the burden of programming onto the computer.
As the years went by and computers got even more powerful, new, higher-level languages started to appear. Examples of these languages are FORTRAN, COBOL, Pascal, and C. With these languages came a programming methodology known as structured programming. Structured programming helped to simplify the systems being designed by allowing programmers to break the problem into small pieces and then implement these pieces as functions or procedures in whatever language was being used.
The structured programming approach worked well for small to medium-sized software applications, but it started to fall apart as systems reached a certain size. OOP tried to solve some of the problems that structured programming was causing. It did this by extending some of the structured programming concepts and by introducing some of its own.
The main concepts that OOP focuses on are the following:
Data Encapsulation
In structured programming, problems often arose wherever there was a data structure that was common to several different pieces of code. One piece of code could access that data without the other piece of code being aware that anything was happening.
Data encapsulation is a process of grouping common data together, storing it into a data type, and providing a consistent interface to that data. This ensures that no one can access that data without going through the user interface that has been defined for that data.
The biggest benefit that this kind of mechanism provides is that it protects code outside the code that is directly managing this data from being affected if the structure of the data changes. This greatly reduces the complexity of large software systems.
C++ implements data encapsulation through the use of classes.
Inheritance
Inheritance is a form of code reuse in which you can inherit or use the data and behavior of other pieces of code. Inheritance is typically used only when a piece of software logically has many of the same characteristics as another piece of software, such as when one object is a specialization of another object.
Inheritance is implemented in C++ by allowing objects to be subclassed by other objects.
Polymorphism
Polymorphism occurs when a language allows you to define functions that perform different operations on objects depending on their type. The true power of this lies in the fact that you can send a message to a base class and that message can be passed down to each of its subclasses and mean different things to each of them.
Polymorphism is implemented in C++ using virtual functions.
In C++, classes can be thought of as C structures that contain not only the data fields but also operations that can be performed on those data fields. A simple example of this concept is a geometric shape. A geometric shape can be many things, such as a rectangle, a triangle, or a circle. All geometric shapes have certain attributes in common, including area and volume. You could define a structure in C called shape in the following way:
struct shape{
float area;
float volume;
}
If you add some common behavior to this structure, you have the equivalent of a C++ class. This would be written as follows:
class shape {
public:
float area;
float volume;
float calc_area();
float calc_volume():
};
You have now defined a C++ class. The calc_area and calc_volume items are known as methods of the class (instead of functions, as in C). Suppose you were to define a variable that was of type shape:
shape circle;
You would have created a circle object. An object is an instance of a class or a variable that is defined to be of the type of a class.
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